A chain gun is a type of machine gun or automatic cannon that uses an external source of power, rather than diverting energy from the cartridge, to cycle the weapon, and does so via a continuous loop of chain similar to that used on a motor or bicycle.

A commonly used chain gun is the M242 Bushmaster. Versions of its 25 mm action are found on ships (the Mk38), Infantry fighting vehicles (the M2 Bradley), and LAV-25 around the world. Others are the M230 30 mm Cannon, which is standard equipment on the Apache helicopter, as well as the Bushmaster II 30 mm, and the Bushmaster III 35/50 mm Chain gun.

A chain gun has a single barrel while a Gatling gun has several rotating barrels. It is a common error to refer to Gatling guns as chain guns, particularly in connection with entertainment such as video games.